1. Field of the Invention
The present invention generally relates to a method and an apparatus for generating periodic patterns and, more particularly, to a method and an apparatus for generating periodic patterns by step-and-align interference lithography, using at least two coherent light beams with controlled intensity distribution to project onto a photo-resist coated substrate to form an interference-patterned region on the substrate. Thereafter, by means of moving the substrate or the light beams stepwisely, a large area composed of continuous patterned regions can be formed on the substrate.
2. Description of the Prior Art
Interference lithography uses at least two light beams to overlap on a substrate so as to form periodic patterns on the photoresist layer on the substrate. Thereby, periodic micro-scale structures such as lines, holes, rods and the like can be manufactured. Using interference lithography, short wavelength light sources and photo-resist, instead of conventional photo-lithography equipments and photo-masks, are required to obtain periodic patterns with small line width, and enables large depth of focus. Therefore, interference lithography has been widely used in various applications such as manufacturing of Bragg gratings for optical fiber communication, opto-electronics and semiconductor light sources such as distributed feedback (DFB) lasers, distributed Bragg reflector (DBR) lasers and photonic crystal structures. Furthermore, interference lithography can also be used along with thermal treatment on a thin film to re-grow the grains and magnetize the grids on a magnetic thin film for data storage applications.
Recently, with the increasing demand of large-area applications in displays, flexible electronic devices and solar cells, it is required that opto-electronic devices are manufactured with a large area. Most of these devices are formed of periodic sub-micro structures. For example, one-dimensional grating structures are used as polarizers for liquid crystal displays. Two-dimensional periodic structures are used as anti-reflection layers to enhance the transmission efficiency of solar cells, and light uniformity of backlight modules in display devices. Therefore, it is crucial to efficiently manufacture large-area periodic structures for consumer electronics industries, and lots of efforts have been made on research and development of large-area periodic structures.
Conventional interference lithography is used for generating small-area periodic sub-micro structures due to the limitation of the optical configuration. In the prior art, Andreas Gombert, et al in Fraunhofer Institute use interference lithography to manufacture large-area periodic sub-micro structures, which is disclosed in “Large-area origination and replication of microstructures with optical functions,” SPIE Vol. 5454, pp. 129, 2004 and “Some application cases and related manufacturing techniques for optically functional microstructures on large areas,” Opt. Eng. 43 (11) 2525-2533, 2004. As shown in FIG. 1, a one-step exposure is used to manufacture periodic structures. The interference lithography system uses a laser 10 to generate a light beam 100, which is split by a beam splitter 11 and reflected by reflectors 12 to generate two coherent light beams 101 and 102. The coherent light beams 101 and 102 overlap on a substrate 14 with a photo-resist layer thereon after passing through lenses 13 to form an interference-patterned region.
However, in FIG. 1, a large space is required for implementation because it takes a distance (21 meters as disclosed by Andreas Gombert, et al.) long enough for the spherical wave from the point light source to travel and expand to an approximate planar wave. Moreover, it takes a considerably long exposure time (for hours) to expose the photo-resist because the power per unit area decreases with the distance. Accordingly, the environment has to be precisely controlled during exposure to prevent disturbance; therefore the exposure conditions such as temperature gradient, airflow, humidity and mechanical vibration need to be stabilized.
Moreover, U.S. Pat. No. 6,882,477 discloses a method and system for interference lithography utilizing phase-locked scanning beams, wherein large-size periodic patterns are formed by partially overlapping sequential boustrophedonic scans of a Gaussian beam. However, in this method, high-precision positioning control is required and the process time is long. Moreover, since the light intensity profile is a Gaussian distribution, each of the overlapped regions has to occupy 60% of a previous scanning region so as to achieve pattern uniformity.
Furthermore, U.S. Patent Pub. No. 2007/0023692 discloses an interference lithography method, in which an alignment window is formed by exposing a photo-resist layer on a transparent substrate (such as a glass substrate or a quartz substrate) to form large-area interference patterns by overlapping sequential scans through an alignment means under the transparent substrate. However, since the light intensity profile is a circular Gaussian distribution, uniform exposure dosage is hard to achieve because the areas of overlapped regions after multi-exposure processing are varied. Moreover, selectivity in photo-resist materials is limited, and the use of the transparent substrate makes the method less compatible with the existing Si-based semiconductor industry.